It is known to perform bone velocity measurement of a subject externally of the subject. Systems typically assume that bones of interest have isotropic propagation characteristics in a plane perpendicular to the longitudinal axis of the bone. U.S. Pat. No. 4,421,119, issued for an invention of Pratti Jr., teaches the use of a pair of opposed transducers for bone velocity measurement. A first transducer is used as a transmitter, and a second transducer is used as a receiver. The transit time of a sound pulse from the transmitter to the receiver can be used to determine bone velocity. It is known to use both continuous waves and pulsed waves to drive the transmitting transducer. Ashman, et al., "A Continuous Wave Technique for the Measurement of the Elastic Properties of Cortical Bone," 17 J. Biomechanics 349-361, 353 (No. 5) (1964). The continuous wave method has been viewed as more accurate, because with pulse propagation, a certain amount of error is introduced in reading the start of the received signal. With the continuous wave approach, however, the phase shift between the two signals can be used to determine the time delay of propagation. Id. However, determining the phase shift can involve ambiguity in the number of periods of delay, which can be resolved by determination of the phase shift separately and a number of different frequencies, graphically plotting the delay, and finding the appropriate intercept of the plot. Id., at 360-361.
In operation of either pulses or the continuous wave approach, furthermore, there is inherent difficulty in dealing with multipath. That is, there is typically more than one path for a waveform to reach the receiving transducer. Finding the transmitted waveform over the path of choice at the receiver poses difficulties for the designer.